What is Migration?
Migration is a natural process in which some animals travel great distances to take advantage of seasonal resources in different locations throughout the year. Although many types of animals can be migratory, the Neotropical Migration Corridor Project is focused on neotropical migratory birds–those which travel throughout the Americas.
The resources in the tropics are constantly available throughout the year, so why do migratory birds travel so far?
When resources are constant throughout the year, so is competition. In more temperate areas, however, the winter makes it difficult for as many animals to survive throughout the year–thus competition is reduced–while the summer in these areas includes proportionally more resources than the tropics, especially:
So that migratory birds can use the abundant resources to reproduce during the summer in North America and return to South America to avoid winter (and take advantage of the abundant resources that the summer in South America offers!).
Types of Migration
Every species, and sometimes different populations, can migrate different distances or use different routes. There are four general approaches to migration, however:
Permanent Residents stay in more or less the same area throughout the year. Most species in Panama belong to this category.
©ryanacandee on Flickr
Short-distance Migrants move with the seasons, but not very far. These movements are sometimes altitudinal, such as that of the three-wattled bellbird, Procnias tricarunculatus, which breeds in the highlands of western Panama but spends its nonbreeding time in the foothills.
Medium-distance Migrants travel further than short-distance migrants, but generally don't leave the continent. Killdeers, Charadrius vociferus, are a good example: they move south after breeding in Canada, but only to the southern United States.
Long-distance Migrants are the focus of this project. Some travel more than 8,000 kilometers each year!
How Does it Work?
What Triggers Migration
We actually don't know what triggers migration, although some patterns are evident. It seems that birds are genetically programmed to depart to or from their breeding grounds, depending on the time of year and amount of daylight, however, they may leave earlier if conditions (for example, temperature or amount of food or competition) become unsuitable.
The time it takes for a bird to migrate depends on the distance it needs to traverse. Species which don't need to travel too far can migrate in a few weeks, while others take four months. Curiously, spring migration are usually faster than in the autumn because birds speed up as they near their breeding grounds.
Most birds migrate at night to take advantage of the cooler temperatures and calm air, with the exception of raptors which use daytime air currents to help them travel.
©Gary Leavens on Flickr
In a similar manner to what triggers migration, we don't entirely understand how birds navigate during their migration. It's clear that the genetics of each species plays an important role, but the role of other learned behaviors is less clear. It's been shown that some species use, in different combinations:
- The Sun
- Mountain Ranges
- Magnetic Fields
Stopovers and Migration Traps
Birds cannot generally migrate in a single flight, thus they need stopover sites where they can rest and eat to prepare for the next part of their migration. Because resources change throughout the year–plants have different flowering and fruiting seasons, for example–birds usually use different stopovers in different seasons.
On the other hand, migration traps are areas which contain especially large populations of migratory birds. These traps can be temporary, such as when a storm forces a bunch of birds to the nearest piece of land, or a combined product of migration routes and geography.
Panama is of special importance for these areas because of its particular location. Almost all migratory routes rejoin in Panama, and for birds coming from the Atlantic side it offers a vital place to rest, either before or after a long journey through the Caribbean.